1. Field
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive demodulating a time-based servo pattern.
2. Description of the Related Art
A disk drive comprises a head attached to a distal end of an actuator arm which is rotated about a pivot to actuate the head radially over a disk. The disk typically comprises embedded servo sectors including head positioning information used to seek the head to a target track and maintain the head over the track during write/read operations. FIG. 1A shows a prior art format of a disk 2 comprising a plurality of embedded servo sectors 40-4N that define a plurality of radially spaced, concentric servo tracks 6. Each servo sector 4i comprises a preamble 8 for synchronizing timing recovery to the data rate, and a sync mark 10 for symbol synchronizing to a servo data field 12, where the servo data field 12 typically comprises a servo track address that identifies the servo track and provides coarse head position information during seeking. Each servo sector 4i further comprises fine head positioning information used to maintain the head over a data track during a tracking operation. In the example of FIG. 1A, the fine head position information comprises a plurality of servo bursts 14 positioned at precise offsets and intervals with respect to the centerline of the servo track. Each servo burst 14 comprises a burst of high frequency magnetic transitions which are typically demodulated by integrating the rectified read signal as the head passes over each burst. The demodulated bursts are then processed mathematically (e.g., compared) to generate a position error signal representing the position error of the head with respect to the servo track.
FIG. 1B illustrates an alternative prior art “time-based” method for generating the position error signal used for centerline tracking. Each servo sector 4i comprises a time-based servo pattern including a plurality of timing marks formed from one or more magnetic transitions (e.g., a plurality of short sync marks). A plurality of time intervals are detected between the timing marks and the position error signal generated in response to the detected time intervals. In the example shown in FIG. 1B, the timing marks form an “open N” shape, wherein the time intervals T1 and T2 change relative to the radial position of the head. To avoid interference between adjacent servo patterns (in the radial direction), the servo patterns are separated circumferentially, as illustrated in FIG. 1B, and processed in an interleaved manner. However, when using the time-based servo pattern shown in FIG. 1B, there is signal loss when the head 16 is positioned between the tracks of adjacent servo patterns. For example, when the head 16 is positioned between track N and track N+1, there is signal loss since the head 16 extends beyond the servo pattern (beyond the bottom of the servo pattern 15i of servo sector 4i and beyond the top of the servo pattern 17i of servo sector 4i+1).
There is, therefore, a need for an improved time-based servo pattern format for use in a disk drive.